IIRC, user count / population was very high and users were likely to have payment methods we could accept, and $1/year is not a significant amount for most residents of Spain. I don't remember if maybe Spain had a high voluntary payment rate too?
The US never had a high user count, but it was chosen because US tech journalism sets the narrative. If you want people to pay around the world, convince US tech journalists that payment enforcement is on, and the knowledge that you need to pay filters through the world in a way that it doesn't by just enforcing payment in Spain.
See also: the invisibility of Nokia phones when they pissed off US carriers with SIP clients and left the US market; despite being the top selling phone manufacture of both feature phones and smart phones, there were no media stories about them.
The US never had a high user count, but it was chosen because US tech journalism sets the narrative. If you want people to pay around the world, convince US tech journalists that payment enforcement is on, and the knowledge that you need to pay filters through the world in a way that it doesn't by just enforcing payment in Spain.
See also: the invisibility of Nokia phones when they pissed off US carriers with SIP clients and left the US market; despite being the top selling phone manufacture of both feature phones and smart phones, there were no media stories about them.